Significantly improved peak rates up to 1 Gbps in the downlink and 500 Mbps in the uplink are required for a Long Term Evolution-Advanced (LTE-A) system as compared to a Long Term Evolution (LTE) system. Also good compatibility of the LTE-A system with the LTE system is required. Carrier Aggregation (CA) is introduced into the LTE-A system so as to accommodate the required improved peak rates, compatibility with the LTE system and full use of spectrum resources.
Carrier aggregation refers to a mechanism in which a User Equipment (UE) can aggregate a plurality of cells concurrently and the plurality of cells can provide the UE concurrently with a data transmission service. In the system with carrier aggregation, carriers corresponding to the respective cells may be consecutive or inconsecutive in the frequency domain, the maximum bandwidth of each component carrier is 20 MHz for compatibility with the LTE system, and there is a bandwidth which may be the same or different across the respective component carriers.
With carrier aggregation, operating cells of a user equipment are categorized into a primary cell (PCell) and several secondary cells (SCells), where the primary cell is responsible for transmission of the majority of control signaling, for example, sending uplink feedback information to downlink data, reporting a Channel Quality Indicator (CQI), transmitting an uplink pilot, etc., and the secondary cell is primarily a resource and responsible for transmitting data.
Random accesses in the LTE system are categorized into a contention-free random access and a contention based random access.
FIG. 1 illustrates a contention-free random access procedure which generally includes the following three steps:
Message 0 (Msg0): Abuse station assigns a UE with a dedicated Random Access Preamble Index (ra-PreambleIndex) for a contention-free random access and the mask index of a Physical Random Access Channel (PRACH) (ra-PRACH-MaskIndex) for the random access; and for a contention-free random access due to downlink data arrival, such information is carried over a Physical Downlink Control Channel (PDCCH), and for a contention-free random access due to a handover, such information is carried in Radio Resource Control (RRC) signaling.
Message 1 (Msg1): The UE sends the specified dedicated preamble to the base station over the specified PRACH resource according to the ra-PreambleIndex and the ra-PRACH-MaskIndex indicated by the Msg0. The base station calculates an uplink Timing Advance (TA) according to the Msg1 upon reception of the Msg1.
Message 2 (Msg2): The base station sends to the UE a random access response including information on the TA and a UL grant (uplink scheduling signaling) to allocate a resource for subsequent uplink transmission, where the TA is used by the UE to subsequently determine a timing relationship of uplink transmission. The PDCCH carrying the Msg2 is scrambled with a Random Access Channel Radio Network Temporary Identify (RA-RNTI) uniquely corresponding to a time-frequency resource over which the Msg1 is sent in a 10 ms window; and moreover the Msg2 further carries a preamble ID, and the UE determines from the RA-RNTI and the preamble ID that the Msg2 is a message corresponding to the Msg1 sent from the UE.
FIG. 2 illustrates a contention based random access procedure which generally includes the following four steps:
Message 1 (Msg1): A UE selects a random access preamble and a PRACH resource and sends the selected random access preamble to a base station over the PRACH resource;
Message 2 (Msg2): The base station receives the preamble, calculates a TA and sends to the UE a random access response including at least information on the TA e and a grant for a message 3 (Msg3);
Message 3 (Msg3): The UE sends uplink transmission over a resource specified by the UL grant in the Msg2, and contents of the uplink transmission of the Msg3 vary from one random access reason to another, for example, an RRC Connection Establishment Request is sent in the Msg3 for an initial access; and
Message 4 (Msg4): The base station sends a Contention Resolution message to the UE, and the UE can judge from the Msg4 whether the random access succeeds.
For both a contention based random access or a contention-free random access, the Msg1 has to be sent at such an instant that refers to the starting point of a downlink radio frame of a cell, and as illustrated in FIG. 3, the user equipment determines the starting point of a downlink radio frame i of a cell and adjusts somewhat the instant at which the Msg1 is sent by the timing advance with reference to the starting point, where the timing advance adjustment amount of the Msg1 is (NTA+NTA offset)×Ts, where NTA is the timing advance adjustment amount used in last timing adjustment, and NTA takes the value of 0 for the Msg1 of a random access; and NTA offset is an offset related to a duplex mechanism, with NTA offset=0 for Frequency Division Duplex (FDD) and NTA offset=624 for Time Division Duplex (TDD). For the Release 10 (R10), multi-TA is not supported, so a random access will be initiated only in a PCell, and the user equipment obtains an uplink timing advance with reference to timing of a downlink carrier of the PCell.
Due to the introduction of carrier aggregation, if cells operating over different carriers have significantly different frequency characteristics and distances between transmitters and receivers, then there may be different uplink timing advances for the different carriers.
Two scenarios in support of multi-TA are currently defined in the 3rd Generation Partnership Project (3GPP).
The first scenario is a scenario in which a Remote Radio Head (RRH) is introduced as illustrated in FIG. 4.
For example, a large coverage area is provided at F1, an RRH is used at F2 for hotspot coverage in an F1 cell, and mobility management is performed based upon F1. In this scenario, if a UE is located in an area where an RRH cell of F2 and the F1 cell overlap, then the F1 cell and the F2 cell can be aggregated, but there are different uplink TAs (UL TAs) for the F1 cell and the F2 cell.
The second scenario is a scenario in which a repeater is introduced as illustrated in FIG. 5.
For example, a base station supports F1 with a large coverage area and F2 with a small coverage area, and the coverage area of F2 can be extended by a frequency selective repeater. In this scenario, if UE is located in an area where the F1 cell and the F2 cell overlap, then the F1 cell and the F2 cell can be aggregated, but there are different UL TAs for the F1 cell and the F2 cell.
In order to facilitate maintenance of the TAs in the multi-TA system, the concept of TA group is introduced, and the same TA can be used for Uplink Component Carriers (UL CCs) of cells belonging to the same TA group, and there are different TAs for UL CCs of cells belonging to different TA groups. In a TA group, the UE will enable uplink synchronization with all of cells in the TA group simply by maintaining uplink synchronization with one of the cells.
The inventors have identified during making of the invention the following technical problem in the prior art:
In the existing R10 specification, only one TA is supported, and timing of an uplink timing advance refers to a downlink carrier of a PCell. With the introduction of a multi-TA scenario, the timing advance of the PCell may be different from those of other Scells, so it is necessary to consider the issue of how to perform uplink transmission in a SCell when the SCell is newly added.